Several types of cables exist for monitoring environmental conditions, sending communications and providing electrical power within a hazardous environment, such as a downhole application in an oil or gas well. One type is a “tube encased conductor” (TEC) cable. A TEC cable characteristically has an electrical conductor disposed within a metal tube for protecting the conductor from the hazardous environment. TEC cables are used for transmitting electrical power to various devices at the distal end of the cable that monitor conditions in the downhole environment, such as temperature and pressure.
Another type of cable for hazardous environments is a “tube encased fiber” (TEF) cable. Similar to TEC cables, TEF cables protect an optical fiber from the environment by disposing the fiber within a small metal tube (referred to as “fiber in metal tube” or “FIMT”). TEF cables may be used in oil and gas wells either as fiber waveguides to transfer data from downhole tools or as sensors for distributed temperature sensing (DTS distributed acoustical sensing (DAS), and other sensing applications.
When the cable contains both electrical conductors and optical fibers encased in one or more tubes, the cable is referred to as a TEC/TEF cable. Commercially available TEC/TEF downhole cables can have a ¼ inch (e.g., 6.35 mm) diameter steel tube as an outer casing. It contains an insulated conductor and another steel tube encasing an optical fiber. TEC/TEF cables of ¼ inch diameter are typically used for permanent installation in an oil well.
As known from, for example, the internet site http://petrowiki.org/Coiled_tubing_drilling, coiled tubing surveys and coiled tubing drilling are also common applications for downhole cables. The term “coiled tubing” refers to a long metal tube, normally 2.5 cm to 8.25 cm in diameter, which is spooled on a large reel. The tube is used for interventions in oil and gas wells and sometimes as production tubing in depleted gas wells. The term “coiled tubing drilling” refers to a combination of coiled tubing and directional drilling (e.g., drilling non-vertical wells) using a mud motor to create a system for drilling reservoirs.
An emerging technique for coiled tubing is to profile an oil well with distributed temperature sensor (DTS) or data acquisition system (DAS) techniques using fiber optic sensors within a TEF cable. Small diameter TEF cables, i.e., cables having an outer diameter of no more than 4 mm, can be injected into the coiled tubing string before the coiled tubing is lowered into the oil or gas well.
To further enhance the well profile, electromechanical pressure sensors may be used when sensing with coiled tubing. Those sensors, however, require electrical current not provided by a TEF cable. The stainless steel tube as part of the FIMT within a TEF cable generally does not have sufficient current carrying capacity because its resistance is too high. Commercially available TEC/TEF cables, which do provide both fiber sensing and electrical current, are too large and heavy for deployment in coiled tubing with their ¼ inch (6.35 mm) diameters. Field experiences with TEF cables for coiled tubing deployments have demonstrated that the largest outer diameter for sensing cables that can be used is 4 mm.
U.S. Pat. No. 8,295,665 (the '665 patent) discloses a downhole hybrid type cable including a center fiber/gel filled stainless steel tube with a copper wire wrapped around the tube and an insulation layer around the copper wire tube. The copper wire is disposed in the helical space formed by the metal tube. The metal tube and the copper element are put into a metallic tube. The metallic tube has a ¼ inch diameter (6.35 mm).
U.S. Pat. No. 5,493,626 (the '626 patent) discloses a downhole electrical/optical instrument cable for use in a well logging system for high-pressure environments. The cable includes a single, hermetically sealed optical fiber for signaling surrounded by layers of protective material and a gel and encapsulated by a protective sheath. The sheath may be a laser-welded metal tube. A layer of electrical conductors between an optional inner insulator and an outer insulator layer surrounds the protective tube and is formed of braided copper rather than a helical “serve” of copper. A plurality of strength member strands surrounds the outer insulator layer. The strength members include an inner layer of stainless steel strands wound helically around the outer insulator layer in one direction, and an outer layer of stainless steel strands wound helically around the inner layer of strength member strands in an opposite serve or winding. The strands and the copper braid layer are conductive and can provide an electrical power supply loop. The total diameter of the cable is approximately 5.77 mm, but it can vary within a range from about 4.76 mm to about 7.94 mm.
U.S. Pat. No. 8,931,549 (the '549 patent) discloses a cable for well logging in marine-submersible and subterranean oil and gas wells. The '549 patent concludes that conventional logging cables with wrapped steel wires and solid copper conductors are not sufficient for deep offshore wells. The disclosed cable includes at least one optical fiber encapsulated in a polymeric material wherein the optical fiber cable is loosely disposed inside a beryllium alloy tube. The beryllium alloy tube, which is conductive, is encapsulated in an amorphous dielectric material, which is further encapsulated on its outer surface by an amorphous polymeric electrically conductive material. This outer layer can be zinc, tin, or other material wrapped, sputtered, or doped the surface to form a shield for both mechanical and electromagnetic effects.
PCT International Publication WO 2009/143461 (the '461 publication) discloses cables for use in a downhole environment, such as in oil or gas wells for conveying well logging tools. In particular, the '461 publication discloses a cable including a cylindrical central core preferably formed of a communication element capable of carrying data signals, such as an optical fiber, which may be encased in a protective metal tube. The cable then includes concentric layers intended to protect a polymer fiber layer in the protective tube. It is preferred that at least one, and possibly both, of inner and outer layers around the polymer fiber layer be formed of a solid electrical conductor, such as a metallic conductor. A layer inner to the polymer fiber layer may be unnecessary if the core is designed to eliminate gas, water, and corrosive migration up and down the core by adding a “water block” agent or fluid. In the event that either of the inner layer and outer layer is not formed of a metallic material, then that layer will preferably be formed of a plastic material such as polyether ethyl ketone (PEEK), or another high density polypropylene. Outer protective sheath will again preferably be formed of PEEK, or another plastic material having exceptional resistance to abrasion, temperature and invasive materials. The '461 publication discloses that cables with an outer diameter of roughly between 0.3 inch and 0.5 inch (about 7-13 mm), will benefit most from this construction.
European Patent Publication EP 0945876 (the '876 publication) discloses a hybrid cable for installation in conduits for fluid media (for example in waste water, fresh water, or gas lines), with at least one optical waveguide arranged in a protective sheath, one or more electrical conductors, and a jacket surrounding the electrical conductor and the protective sheath. In particular, the '876 publication discloses a hybrid cable with two concentric metal tubes, of which at least the external one is corrugated. An insulating layer separates the two metal tubes, and a jacket of polyethylene surrounds the external tube. The inner tube surrounds a plurality of optical fibres. In many cases, conductivity of the inner metal tube can be sufficiently achieved by coating its surface with a metal of high conductivity.
PCT International Publication WO 2015/038150 (the '150 publication) discloses a fiber optic electrical core that may be incorporated into a fiber optic slickline (application that is run over a conveyance line that is substantially below about 0.25 inches, i.e. 6.35 mm, in overall outer diameter). One or more fiber optic threads, each jacketed by a conventional polymeric buffer, may be placed within a welded steel tube in a loose fashion with a sufficiently thick electrically insulating polymer layer thereabout, and surrounded by a conductive member. The electrically conductive member may also be surrounded by an insulating polymer jacket. To complete the fiber optic slickline, the fiber optic electrical core may be surrounded by a synthetic fiber layer. Adherence between a subsequent cladding layer and the fiber synthetic layer may be enhanced by way of the intervening adherent layer. Cladding layer may be a conventional metal-based layer such as a steel jacket.
Applicant has faced the problem of providing a TEC/TEF downhole cable with minimal diameter, particularly with a diameter of no more than 4 mm suitable for coiled tubing drilling applications. Those cables need to be capable of insertion into a coiled tubing string before the coiled tubing is lowered into the oil or gas well. TEC/TEF cables designed for well logging applications are too large, typically having an outer diameter of 6.35 mm, which can be difficult to insert in a coiled tubing and for limited length (not greater than 3 km). While some TEF cables having diameters within 4 mm are known, those small-diameter TEF cables do not have electrical conductors, nor do they have sufficient current carrying capacity due to the high electrical resistance of theft stainless steel tube.